The invention relates to the domain of ring furnaces for baking blocks containing carbon and more particularly a process and a device for regulation of these furnaces.
Regulation methods for this type of furnace are already known, for example as described in French applications FR 2 600 152 and FR 2 614 093 submitted by the Applicant, and in international application WO 91/19147.
This type of furnace, also called an xe2x80x9copen sectionxe2x80x9d furnace comprises several preheating, baking and cooling sections in the longitudinal direction (as described in the referenced documents), the composition of each section in the transverse direction consisting of flue walls through which combustion gases circulate alternating with pits in which blocks containing carbon to be baked are stacked, the blocks being immersed in dust containing carbon.
This type of furnace comprises two bays whose total length may exceed a hundred meters. Each bay comprises a series of sections separated by head walls and open in their upper part, through which unbaked blocks are loaded and cooled baked blocks are unloaded. Each section includes a set of thin flue walls parallel to the longitudinal direction of the furnace, in other words its major axis, through which the hot gases or combustion exhaust gases which provide the heat for baking will circulate, alternating in the transverse direction of the furnace with pits in which the blocks to be baked are stacked.
Closable openings called xe2x80x9cpeep holesxe2x80x9d are placed in the upper part of the flue walls. They are also provided with baffles to extend and more uniformly distribute the trajectory of combustion gases or exhaust gases.
The furnace is heated by burner ramps, the length of which is equal to the width of the sections, the injectors for these burners being inserted through peep holes in the flue walls of the sections concerned. On the upstream side of the burners (upstream considering the direction in which combustion is advancing), combustion air blowing openings are placed on an air blowing ramp equipped with fans, these blowing openings being connected to the said flue walls through the peep holes. On the downstream side of the burners, combustion exhaust gas openings are installed on an exhaust ramp supplying the exhaust gas collection centers equipped with dampers which close off the said exhaust openings to the required level. Heating is applied by combustion of the fuel injected in the baking sections, and by combustion of tar vapor released from the blocks during baking in the preheating sections, which due to the negative pressure in the preheating sections, leaves the pits by passing through the flue wall and burns with the oxygen remaining in the combustion exhaust gases circulating in the flue walls in these sections.
Typically, there are about ten sections xe2x80x9cactivexe2x80x9d at the same time; four in the cooling area, three in the heating area and three in the preheating area.
As baking continues, the xe2x80x9cblowing openingsxe2x80x94burnersxe2x80x94exhaust openingsxe2x80x9d assembly will be moved forward by one section, for example every 24 hours, the sequence of operations in each section consisting of loading an unbaked block containing carbon in front of the preheating zone, then natural preheating in the preheating zone due to combustion exhaust gases and combustion of tar vapors, then heating the blocks to 1100-1200xc2x0 C. in the baking zone, and finally cooling the blocks by cold air in the cooling zone at the same time as preheating combustion air for the furnace, the cooling zone being followed by a zone in which the cooled blocks containing carbon are unloaded.
The most frequently used method of regulation for this type of furnace is to regulate the temperature and/or pressure in a number of sections in the furnace. Typically, out of the ten sections that are active at any one time, four will be provided with temperature measurements and two will be provided with pressure measurements. Firstly, the three burner ramps are regulated as a function of the temperature of the combustion exhaust gases, the fuel injection being adjusted to follow a temperature rise curve (typically the temperature of the combustion exhaust gases but possibly the temperature of the blocks containing carbon). Secondly, the fan speed on the air blowing ramp is typically regulated as a function of a static pressure measured on the upstream side of the burners, but it may also be kept constant. Finally, the exhaust gas dampers are regulated as a function of a negative pressure measured in a section located between the burners and the exhaust openings. But more frequently (particularly in more recent furnaces) the said negative pressure is itself controlled by a set temperature, which is typically the temperature of combustion exhaust gases such that the said dampers are controlled by a temperature measurement and its comparison with a set temperature.
The furnace may also be regulated by other complementary means:
French application FR 2 600 152 also describes a device for optimizing combustion in the baking area in order to measure the opacity of exhaust gases in the exhaust openings and to regulate this exhaust correspondingly;
French application FR 2 614 093 also describes a method of optimizing combustion in the furnace by continuously injecting the necessary and sufficient air quantity to obtain complete combustion of volatile materials released during baking of the blocks containing carbon and the fuel injected in the burners;
application WO 91/19147 also describes a check on the oxygen/fuel ratio in the furnace by measuring the oxygen content in the furnace.
Regulation methods used in the past are based mainly on temperature measurements and pressure measurements in a large number of sections, and in the various flue walls in the same section. As indicated in the mentioned state of prior art, these basic measurements may be complemented by other measurements.
Furthermore, temperature and pressure set values are known for each section, and must be respected so that the quality of the resulting blocks containing carbon is satisfactory and to ensure that the furnace operates correctly, particularly in the preheating area. Volatile materials contained in the tar escape while the blocks containing carbon to be baked are being preheated. It is important that these gases or vapors are drawn in towards the flue walls and burn immediately in the presence of the residual oxygen present in combustion exhaust gases. Otherwise, these tar vapors could form a deposit on the openings, the exhaust ramp and pipes leading to the collection system. These deposits can ignite on contact with incandescent particles of carbon dust. These fires damage flues and their hot exhaust gases burn the filters and fans in collection centers. Considering these risks, safety margins are adopted by increasing the flows of drawn in combustion exhaust gases, which in turn cause excess fuel consumption and reduce the energy performances of the furnace.
Furthermore, it is observed that current regulation of furnaces results in instabilities and generates sudden random variations in the flows of drawn in combustion exhaust gases and fuel flows, such that heat transfer conditions in the furnace are not stable, which has an adverse effect on the efficiency of the heat exchange or heat transfer between the combustion exhaust gases and the said blocks containing carbon.
Finally, this dispersion of the various flows leads to a dispersion in baking levels which makes it necessary to overbake some of the blocks containing carbon or anodes to guarantee the minimum quality in all anodes, which automatically reduces the energy performances of the furnace.
Finally, the current methods used for furnace operation and regulation are characterized firstly by a considerable increase in the number of measurement sensors, and secondly by adoption of large safety margins for each of the main three parameters used to operate the furnace; blowing air on the upstream side of cooling sections, fuel injection in baking sections, and drawing in combustion exhaust gases on the downstream side of the preheating sections.
The results of this state of affairs are that:
firstly, the complete set of measurement and regulation means form a non-negligible part of the investment and operating costs of the furnace, since many of the sensors have a short life due to the particularly severe temperature and environmental conditions, and consequently can be considered as being consumables,
secondly, since these measurement and regulation means are incapable of stabilizing furnace operation, the result is that energy consumption is variable and the average consumption is significantly greater than the optimum considering safety margins taken to guarantee the quality of the blocks containing carbon made and to guarantee the integrity and durability of the furnace.
This invention is intended to solve these two problems and to operate the furnace automatically and optimally while reducing the investment cost and the operating cost of control and regulation equipment, and the energy consumption of the furnace.
A first object of the invention is a process for regulating a ring furnace for baking blocks containing carbon, and including a sequence of sections Ci that are active simultaneously but in a different manner, namely working along the longitudinal direction from upstream to downstream, cooling sections the first of which at the head is supplied with atmospheric air through blowing openings Sj, baking sections equipped with at least one burner ramp with injectors Ij supplied with fuel, and preheating sections the last of which at the tail is equipped with combustion exhaust gas openings Aj, and in the transverse direction comprising a sequence of flue walls Clij alternating with pits Alij in which blocks containing carbon to be baked are stacked, the said flue walls Clij in a given section Ci being fitted with peep holes through which the said blowing openings Sj and/or the said injectors Ij and/or the said exhaust openings Aj and/or measurement means communicating with flue walls Clixe2x88x921j and Cli+1j in the previous section Cixe2x88x921 and the next section Ci+1 will be fitted, to control circulation of a gaseous stream from the upstream side towards the downstream side, the gas including atmospheric air and/or combustion exhaust gases, characterized in that the mass flow DGj of each of the combustion exhaust gas streams Gj passing through the said exhaust openings Aj at the tail of the preheating sections, is regulated by measuring the mass flow DGj and the temperature Tj of each of the combustion exhaust gas streams Gj, by calculating the corresponding energy fluxes Ej, typically by calculating the product R equal to DGj. (Tjxe2x88x92Ta) Cg. where Tj and Ta are the temperature of the combustion exhaust gases Gj and the ambient air respectively, and Cg is the specific heat of combustion exhaust gases at temperature Tj, so as to maintain the said energy flux Ej equal to a predetermined set value Eoj for each of the combustion exhaust gas streams Gj.
This set value Eoj may either be a predetermined constant, or a predetermined function of time f(t). Typically, mobile furnace equipment (burner ramps, blowing openings ramp, exhaust openings ramp, etc.) is moved forward by one section every 24 hours. Therefore, set values which depend on time are defined over this period T, as may be the case for Eoj. During the time T in which combustion is taking place on a given section, it may be useful to have a set value Eoj which includes either one ramp, in other words a regular variation of the set value Eoj during the residence time, or particular set values at the beginning or end of the residence time T.
Therefore, the essential aspect of the invention is the fact that the energy flux Ej in the combustion exhaust gases drawn in by each exhaust opening Aj is determined in order to control furnace actuators, whereas in prior art the exhaust openings and the burners were controlled as a function of a temperature curve which itself usually depends on time during the period T.
The energy flux Ej in each stream of combustion exhaust gases is actually an enthalpy flux for which a good approximation can be obtained using the value of R equal to (DGj. (Tjxe2x88x92Ta). A more precise value may be obtained by replacing xe2x80x9c(Tjxe2x88x92Ta). Cgxe2x80x9d by the value of the integral ∫Cg(T). dT for T between Ta and Tj, or by any approximate polynomial expression for this integral.
Surprisingly, the applicant found that this means which is an essential part of the invention, solves the problem that arises, even though it is much simpler than control means used in the state of the art. The applicant was able to verify in particular that this means enabled:
stable operation of the furnace, instead of operation with sudden parameter variations,
economic operation, concerning fuel consumption,
simplification of control and regulation equipment and devices.
Globally, the result is the manufacture of blocks containing carbon baked with a more constant quality and at lower cost. The reasons for which the means according to the invention gives these surprising results have not been clearly defined. However, according to one hypothesis made by the applicant, external air streams that penetrate at negative pressure into preheating sections of a furnace with open sections, could interfere with operation of the furnace and cause a disturbing element that accentuates variations in furnace parameters.
Based on this hypothesis, the applicant had the idea of using a regulation parameter independent of the variable added quantity of external air. To do this, he found that a parameter such as the parameter R, equivalent to an energy flux with respect to ambient temperature, was completely independent of the variable quantity of air that entered into the furnace and consequently could enable effective regulation of the furnace with stable and economic furnace operation.
According to the invention, the said set value denoted Eoj of energy fluxes Ej in combustion exhaust gases Gj is chosen, usually experimentally, to be the lowest possible value compatible with standard quality requirements for manufactured blocks containing carbon and furnace operation.
According to the invention, there is no need to regulate all energy fluxes Ej, but a limited number may be regulated, for example every second flux. In this case, flux Ek which is not regulated is considered to be equal to the average of the values of the adjacent regulated fluxes Ekxe2x88x921 and Ek+1.